1. Field of the Invention
The present invention relates to a wired circuit board and, more particularly, to a wired circuit board such as a flexible wired circuit board and a suspension board with circuit.
2. Description of the Prior Art
A wired circuit board, such as a flexible wired circuit board and a suspension board with circuit, comprises in general an insulating base layer of e.g. polyimide resin, a conductive layer of copper foil e.g. formed on the insulating base layer, and an insulating cover layer of e.g. polyimide resin formed on the insulating base layer for covering the conductive layer. This wired circuit board is widely used in various fields of electric and electronic devices and equipment.
Meanwhile, there has been proposed an improvement of this wired circuit board, according to which a thin nickel film is formed on a surface of a conductive copper layer by electroless nickel plating to cover the surface of the conductive copper layer (cf. JP Laid-open (Unexamined) Patent Publication No. Hei 10-12983, for example).
The proposed thin nickel film described in JP Laid-open (Unexamined) Patent Publication No. Hei 10-12983 functions as a barrier layer to cover the surface of the conductive copper layer to prevent the copper (metal) of the conductive copper layer from diffusing into the insulating cover layer, so as to prevent deterioration of the conductive copper layer and short-circuiting of the conductive copper layer.
However, since this proposed thin nickel film described in JP Laid-open (Unexamined) Patent Publication No. Hei 10-12983 is formed by electroless nickel plating, it may be difficult to form the thin nickel film in a uniform thickness. If this is the case, a thinned portion of the thin nickel layer cannot substantially function as the barrier layer so that there is still a possibility that deterioration of the conductive copper layer and short-circuiting of the conductive copper layer may occur.